What it looks like in your logs
550-5.1.1 The email account that you tried to reach does not exist. Please try double-checking the recipient's email address for typos. - gsmtpWhat does 5.1.1 mean?
RFC 3463 defines X.1.1 as bad destination mailbox address. The domain exists, its mail server answered, and that server says there is no mailbox for the part before the @. It is the most common hard bounce in email and the clearest one: the address will not work until someone creates that mailbox, which almost never happens. Retrying is pointless and costs you reputation.
Mailbox providers watch how often you hit unknown users. An occasional 5.1.1 is normal because people leave jobs and abandon accounts. A cluster of them looks like a purchased, scraped or stale list, and it drags down sender reputation quickly. Aim to keep total hard bounces well below 2% of a send, and ideally under 0.5%. Treat any single import or signup source that produces a spike as suspect.
Gmail says "The email account that you tried to reach does not exist". Microsoft 365 says the address could not be found or that the recipient was not found by SMTP address lookup. Yahoo usually returns 554 with "This user doesn't have a yahoo.com account" instead of a 5.1.1 string, but it means the same thing.
Soft or hard bounce?
| Soft bounce (4xx) | Hard bounce (5xx) | |
|---|---|---|
| Nature | Temporary | Permanent |
| Sender behaviour | Queue and retry with backoff | Stop, suppress the address |
| Reputation risk | Low, unless it repeats for days | High above a 2% bounce rate |
| 5.1.1 | ✓ this code |
Common causes
- The address was mistyped at signup, for example gmial.com or a missing letter.
- The account was deleted or closed by the user.
- The employee left and IT removed the mailbox.
- The address never existed and came from a purchased or scraped list.
- The domain previously accepted everything (catch-all) and has since switched it off.
How to fix 5.1.1
- Suppress the address automatically on the first 5.1.1; there is no retry that will work.
- Check for common domain typos and, where you have another contact route, ask the person for a corrected address.
- Run the full list through an email verifier (ZeroBounce, NeverBounce or SendPulse's email verifier) before the next campaign; see email verification tools compared.
- Add real-time validation to signup forms so invalid addresses are rejected on entry.
- Use double opt-in so only confirmed addresses reach your list.
- Track hard bounce rate per send in your bounce reporting and investigate any source that produces spikes.
5.1.1 questions
What does 550 5.1.1 mean?
It means the recipient mailbox does not exist on the destination server. The domain is valid, but that specific user is not. It is a permanent failure, so remove the address from your list and do not send to it again.
Why does Gmail return 5.1.1 when I know the user exists?
Check the address carefully first: a single wrong character is the usual answer. If it is exactly right, the account may have been deleted or suspended, or the person may be using a different address than you think. Gmail does not return 5.1.1 for policy blocks, so the mailbox is genuinely not there.
Does a 5.1.1 bounce hurt my sender reputation?
A few do not. Many do. Providers treat a high rate of unknown-user bounces as evidence of poor list practices, and it can push you toward the spam folder or into throttling. Keep hard bounces under 2% and clean your list with our list hygiene guide.
Should I remove 5.1.1 addresses from my list?
Yes, immediately and permanently. Most ESPs do this for you by adding the address to a suppression list. Do not reimport those addresses later from an old export, which is a common way senders reintroduce dead mailboxes. Keep them suppressed forever.